1) Field of the Invention
The present invention relates to a relay transmission apparatus in a network having a function of selecting either of signals transmitted through two different routes as a received signal such as an OUPSR (Optical Unidirectional Path Switched Ring) network.
2) Description of the Related Art
In the communication industry, enterprises such as MSOs (Multi Service Operators) recently have increasing demand for distribution of a large volume of data such as motion picture data as that seen in a cable television broadcast. As a basic network adapted for transmission of such data, an increasing number of enterprises are constructing a wavelength division multiplex (WDM) network, which requires a smaller number of optical fibers and therefore allows a reduction in running cost when compared to the conventional TDM (Time Division Multiplex) network.
When a large volume of data are distributed in the mode of broadcasting mentioned above, a receiving station is required to have not only a drop function for distributing signals from a signal distributing station to the network to which the own receiving station belongs but also a continue function for relay-transmitting the signals from the signal distributing station to another receiving station in the neighborhood of the same (the functions are hereinafter collectively referred to as “Drop and Continue”).
The introduction of the OUPSR method into such a network distributing a large volume of data is being studied in an intention to ensure distribution of a large volume of data and to improve reliability of communication. In order to use such a drop and continue function in a WDM network while achieving OUPSR network topology and redundancy of signals, for example, the network may be configured as shown in FIG. 16.
Here, in a network 100 shown in FIG. 16, a signal distributing station 101 as a head end station and three receiving stations 102 to 104 as drop stations are connected to each other through a bidirectional transmission route 105 to form a ring network which has clockwise (RR) and counterclockwise (LR) as the signal transmitting directions.
Further, the signal distributing station 101 is composed of a data generation unit 101a for generating (a large volume of) data to be distributed to the receiving stations 102 to 104 as a wide-band optical signal, a transponder unit 101b for performing inter-transformation of optical wavelengths between the wide-band optical signal and a narrow-band optical signal which is to be wavelength-multiplexed, and a WDM unit 101c for multiplexing the wavelength of the narrow-band optical signal from the transponder unit 101b to transmit it in a clockwise (RR) transmission route 105 and a counterclockwise (LR) transmission route 105. The WDM unit 101c transmits two data sets from the transponder unit 101b using the same wavelength channel along with a signal in another wavelength channel as wavelength multiplex optical signals.
Further, the receiving stations 102 to 104 have WDM units 102a to 104a, respectively, and also is composed of switches 102b to 104b and data receiving units 106 to 108, respectively. The WDM units 102a to 104a receive the input of wavelength multiplex optical signals from the clockwise (RR) and counterclockwise (LR) transmission routes 105 and perform a required drop-and-continue process for an optical signal in the wavelength channel for distribution of a large volume of data as described above. The switches 102b to 104b selectively output either of the signals from the transmission routes 105 in two directions (RR and LR) dropped at the WDM units 102a to 104a to respective data receiving units 106 to 108 as a received signal.
Let us now discuss about the large volume of data to be distributed from the signal distribution station 101 to each of the receiving stations 102 to 104 in the OUPSR network 100 having such a configuration. Then, the signals (the large volume of data) transmitted in the transmission routes 105 in two directions, i.e., clockwise (RR) and counterclockwise (LR) directions, are dropped and continued at each of the receiving stations 102 to 104.
Specifically, in the route in which a signal is transmitted from the signal distributing station 101 as the starting point to the receiving station 102, the receiving station 103, and the receiving station 104 in the order listed (clockwise route), the signal is dropped and continued at the receiving stations 102 and 103 serving as relay stations. In the route in which a signal is transmitted from the signal distributing station 101 as the starting point to the receiving station 104, the receiving station 103, and the receiving station 102 in the order listed (counterclockwise route), the signal is dropped and continued at the receiving stations 104 and 103 serving as relay stations.
At the respective receiving stations 102 to 104, the WDM units 102a to 104a selectively output (called as being protection function) either of the signals dropped from the routes in two directions, i.e., clockwise and counterclockwise directions as described above depending on switching at the switch units 102b to 104b, which provides redundancy in signal transmission and allows the drop signal having better quality to be output.
For example, the switch unit 103b of the receiving station 103 selectively outputs either of a signal which has followed the clockwise route via the receiving station 102 or a signal which has followed the counterclockwise route via the receiving station 104 to a data receiving unit 107. As thus described, the drop-and-continue function can be used in an OUPSR configuration with redundancy added to the function in order to improve reliability of communication.
Patent Documents 1 and 2 listed below disclose techniques related to this invention.
Patent Document 1 discloses an optical circuit that is provided at a node for distributing an optical signal in a network having a multicast function, the circuit outputting a part of the optical intensity of signal light which has been branched to a drop port and outputting the remaining part of the optical intensity of the signal light to a main output port after adjusting it to a required level.
Patent Document 2 discloses a technique for a light insertion/branching ring system of the wavelength multiplex optical transmission system, in which a wavelength component to be regeneratively relayed among main signal wavelength components separated by a wavelength separating unit are regeneratively relayed according to a bit rate using a bit-rate-selection type regenerative relay device. However, the document discloses no technique for switching drop signals in a network having an OUPSR configuration.
[Patent Document 1] Japanese Patent Laid-Open Publication No. JP-A-2000-89263
[Patent Document 2] Japanese Patent Laid-Open Publication No. JP-A-10-303863
However, at each receiving station of a network like the OUPSR network 100 shown in FIG. 16 having the selection function of any of a plurality of signals transmitted through different routes as a received signal, variation of an OSNR (Optical Signal Noise Ratio) occurs even between identical signals because they are transmitted different distances depending on the routes they follow. Specifically, a signal must normally be amplified many times to be relayed a long transmission distance, which results in deterioration of the ONSR of the signal. A problem therefore arises in that the effectiveness of a protection function provided by OUPSR cannot be improved.
For example, when a drop signal received at the receiving station 102 described above directly received from the signal distributing station 101 through the clockwise route and a drop signal received through the counterclockwise route via the receiving stations 104 and 103 are compared, it is considered that the drop signal following the counterclockwise route is more likely to have a degraded OSNR as described above because of the length of the transmission route. It is therefore impossible to improve the effectiveness of redundant transmission of signals in such an OUPSR network.